Contrast-Dependent Decrease In Task-Transfer Function Area with Strength of Iterative Reconstruction and Deep Learning In Low-Contrast Lesions
Abstract
Purpose
Iterative and deep learning image reconstruction (IR and DLIR) are valuable for reducing dose in modern CT but have the effect of reducing lesion edge sharpness. If sharpness loss becomes contrast dependent at some threshold, the decrease in sharpness may exceed the detectability gained from noise reduction. We propose that for targets with contrast below 40 HU, the rate of edge degradation with increasing IR strength is contrast-dependent.
Methods
A custom anthropomorphic liver phantom, including fifteen <100 HU lesions, was scanned on a GE Revolution CT at 10 mGy CTDIvol. Image sets were reconstructed for ASIR-V 0-100% in 10% increments, and for DLIR strengths low, medium, and high. Fourteen acquisitions were averaged together for each image set. Two line-profiles were drawn through each lesion center at symmetric angles and pixels were projected to oversample the lesion edge. Lesion line profiles were reflected about the lesion center and the edge regions were aligned, combined, and averaged. The resultant edge profile was smoothed using a Savitzky-Golay filter and the gradient determined by central-difference approximation. Fast Fourier transform was used to find the task transfer function, and the area under the curve was used as the spatial resolution metric.
Results
The dependence of edge spatial resolution on IR strength was contrast-independent above 30 HU but increased by 2.4x as contrast decreased to 15 HU. Regarding DLIR, the spatial resolution metric transitioned from “similar to high ASIR-V” to “similar to low ASIR-V” between 40-60 HU.
Conclusion
Noise suppression from IR was observed to be contrast dependent for low-contrast lesion edges. Below 30 HU, spatial resolution loss with increasing IR strength was strongly dependent on contrast. The spatial resolution metric for DLIR results was comparable to high IR (more blurring) below 30 HU but was comparable to low IR (less blurring) above 60 HU.